Mutations in K-Cl cotransporter 3 (KCC3) are responsible for one of the most severe, early onset peripheral neuropathy disorders called ACCPN or "Agenesis of Corpus Callosum associated with Peripheral Neuropathy". We have disrupted the KCC3 gene by homologous recombination in mouse embryonic stem cells and the mouse peripheral nerve phenotype is similar to that observed in ACCPN patients. Due to the difficulty in studying K-Cl cotransport function in peripheral nerves, we propose to further examine the role of KCC3 in ACCPN by creating additional mouse models. First, as all but one human mutation results in truncated KCC3 proteins with shorter carboxyl- termini, we propose to create a knock-in mouse with a single missense amino acid substitution and assess whether absence of cotransporter function or absence of protein-protein interaction is causing the disorder. Second, as the cotransporter is expressed in both sensory neurons and Schwann cells, we propose to create mice with tissue-specific deletions of the cotransporter. To significantly reduce the time necessary to assemble the constructs and increase efficiency in obtaining these new mouse models, we have designed a common modular targeting construct. As we have identified a single residue substitution that renders the cotransporter non-functional without affecting its membrane expression, we will utilize the exon encoding this residue as our targeted region to construct the three mouse lines. In this way, both constructs will contain the same arms of recombination and allow for a common ES cell screening strategy to identify recombination events. Mouse phenotype will be analyzed through locomotor activity and peripheral nerve pathology. These two new mouse models will allow us to further understand the developmental versus degenerative nature of the neuropathy, as well as the molecular basis and cell-type origin of the disorder. PUBLIC HEALTH RELEVANCE The goal of this proposal is the creation of three novel KCC3 mouse models that will advance our understanding of the peripheral neuropathy disorder associated with disruption of the KCC3 gene in humans.